Human diploid fibroblasts with mutations in L-isoleucine uptake and catabolism will be used as tools for the understanding of branched chain amino acid transport and metabolism. In the transport studies, emphasis will be placed on the isolation of a membrane associated isoleucine binding protein. In the biochemical studies, emphasis will be placed on alternate pathways for isoleucine catabolism and on the regulation of enzymes in the pathways for isoleucine and glycine catabolism.